Composite compound with mixed crystalline structure

ABSTRACT

A composite lithium compound having a mixed crystalline structure is provided. Such compound can be formed by heating a lithium, iron, phosphorous and carbon mixed compound with another metal compound together. The resulting mixed metal crystal can exhibit superior electrical property and is a better cathode material for lithium secondary batteries.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 12/040,773 filed Feb. 29, 2008, which is incorporated herein by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The embodiments of the present invention relate to lithium secondary batteries, more specifically, to a composite compound having a mixed crystalline structure that can be used as a cathode material for lithium secondary batteries.

BACKGROUND

Lithium secondary batteries are widely used in various devices such laptops, cameras, camcorders, PDAs, cell phones, iPods and other portable electronic devices. These batteries are also growing in popularity for defense, automotive and aerospace applications because of their high energy density.

Lithium phosphate-based cathode materials for secondary battery have long been known in the battery industry. People have used metal intercalation compound to improve the electrical property of lithium phosphate. One popular intercalation compound is lithium iron phosphate (LiFePO₄). Because of its non-toxicity, excellent thermal stability, safety characteristics and good electrochemical performance, there is a growing demand for rechargeable lithium secondary batteries with LiFePO₄ as the cathode material.

LiFePO₄ has its problems as a cathode material, however. Compared with other cathode materials such as lithium cobaltate, lithium nicklate, and lithium magnate, LiFePO₄ has much lower conductance and electrical density. The current invention solves the problem by producing a mixed crystal structure to significantly enhance the electrical properties of LiFePO₄.

A mixed crystal can sometimes be referred to as a solid solution. It is a crystal containing a second constituent, which fits into and is distributed in the lattice of the host crystal. See IUPAC Compendium of Chemical Terminology 2nd Edition (1997). Mixed crystals have been used in semiconductors for enhancing light output in light emitting diodes (LEDs). They have also been used to produce sodium-based electrolyte for galvanic elements. The current invention is the first time that a mixed crystal has been successfully prepared for lithium metal intercalation compounds such as LiFePO₄. It is also the first time that a mixed crystalline structure has been used as a cathode material for lithium secondary batteries. The new cathode material disclosed in the present invention has significantly better electrical properties than traditional LiFePO₄ cathode materials.

SUMMARY

Accordingly, a first embodiment discloses a method of preparing a cathode active material for lithium secondary batteries comprising: providing a first crystalline substance having one or more members with the general formulas Li_(x)M_(y)(XO₄)_(z), LiMXO₅, LiMXO₆ and LiMX₂O₇, wherein: 0<x/z≦1 and 0≦y/z≦1; M is selected from elements Na, Mn, Fe, Co, Ni, Ti, Nb and V; X is selected from elements P, S, As, Mo and W; providing a second crystalline substance having one or more members with the general formula A_(a)M_(b)N_(c)O_(d), wherein: A, M and N are different metals selected from groups IIA, IIIA, IVA, VA, IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table; 0≦a≦6, 0≦b≦6, 0<c≦6 and 0<d≦12; wherein a and b cannot both be 0 at the same time; and sintering the two crystalline substances at about 500 to 800° C. for about 5 to 32 hours to provide a mixed crystal, the mixed crystal having electrical conductivity of about 0.01 to 10 S/cm at about 25° C.

The first crystalline substance and the second crystalline substance has molar ratios of about 1 to 0.01-0.05. The method further includes sintering a carbon additive with the two crystalline substances, the carbon additive capable of providing the mixed crystal with about 1-5% of carbon by weight. The carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol.

The first crystalline substance includes one or more members selected from the group consisting of LiFePO₄, LiMnPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃, Li₂NaV₂(PO₄)₃, LiTiPO₅, LiVMoO₆, LiVWO₆, LiVP₂O₇ and LiFeAs₂O₇; and wherein the second crystalline substance includes one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆. The second crystalline substance can be produced by heating the oxide compounds of A, M and N at molar ratio of a:b:c at about 400 to 1000° C. for about 8 to 15 hours.

A second embodiment discloses a method of preparing a cathode active material for lithium secondary batteries comprising: providing a first crystalline substance having a combination of lithium, iron and phosphorous sources, wherein the lithium, iron and phosphorous (Li:Fe:P) sources have molar ratios of about 0.95-1.1:1:0.95-1.1; providing a second crystalline substance having one or more members with the general formula A_(a)M_(b)N_(c)O_(d), wherein: A, M and N are different metals selected from groups IIA, IIIA, IVA, VA, IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table; 0≦a≦6, 0≦b≦6, 0<c≦6 and 0<d≦12; wherein a and b cannot both be 0 at the same time; and sintering the two crystalline substances at about 500 to 800° C. for about 5 to 32 hours to provide a mixed crystal, the mixed crystal having electrical conductivity of about 0.01 to 10 S/cm at about 25° C.

The phosphorous source and the second crystalline substance has molar ratios of about 1 to 0.01-0.05, taking only the phosphorous source in the material into consideration. The second crystalline substance can be produced by heating the oxide compounds of A, M and N at molar ratio of a:b:c at about 400 to 1000° C. for about 8 to 15 hours and includes one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆.

The lithium source includes one or more members selected from the group consisting of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate; the iron source includes one or more members selected from the group consisting of ferrous oxalate, ferrous carbonate, iron acetate, iron oxide, iron phosphate, iron pyrophosphate and iron nitrate; and the phosphate source includes one or more members selected from the group consisting of ammonium phosphate, ammonium dihydrogen phosphate, ammonium, iron phosphate, phosphoric acid and lithium dihydrogen phosphate.

The method further includes sintering a carbon additive with the two crystalline substances, the carbon additive capable of providing the mixed crystal with about 1-5% of carbon by weight. The carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol.

A third embodiment discloses a method of preparing a cathode active material for lithium secondary batteries comprising: providing a first crystalline substance having one or more members selected from the group consisting of LiFePO₄, LiMnPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃, Li₂NaV₂(PO₄)₃, LiTiPO₅, LiVMoO₆, LiVWO₆, LiVP₂O₇ and LiFeAs₂O₇; providing a second crystalline substance having one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆; and sintering the two crystalline substances at about 500 to 800° C. for about 5 to 32 hours to provide a mixed crystal, the mixed crystal having electrical conductivity of about 0.01 to 10 S/cm at about 25° C.

The method further includes sintering a carbon additive with the two crystalline substances, the carbon additive capable of providing the mixed crystal with about 1-5% of carbon by weight. The carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol. The second crystalline substance can be produced at temperatures of about 400 to 1000° C. for about 8 to 15 hours.

In other embodiments, batteries may be manufactured using the cathode materials as described in the previously disclosed embodiments.

Other variations, embodiments and features of the present invention will become evident from the following detailed description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 illustrate structural relationships of a mixed crystal, specifically, between a lithium iron phosphate compound and a composite metal compound;

FIG. 5 illustrates an x-ray diffraction (XRD) pattern of a composite compound according to Example 2;

FIG. 6 illustrates the XRD pattern of a composite compound according to Example A1;

FIG. 7 illustrates the XRD pattern of a composite compound according to Example A2;

FIG. 8 illustrates the XRD pattern of a composite compound according to Example A3;

FIG. 9 illustrates the XRD pattern of a composite compound according to Example A4;

FIG. 10 illustrates the XRD pattern of a composite compound according to Example A5; and

FIG. 11 illustrates the XRD pattern of a composite compound according to Example A6.

DETAILED DESCRIPTION

It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive.

A cathode material for lithium secondary batteries can be provided by combining at least one lithium metal compound with at least one mixed metal crystal, wherein the lithium metal compound has an olivine structure and the mixed metal crystal includes a mixture of metal elements and metal oxides.

A general formula for a mixed crystal compound can be expressed as:

Li_(a)A_(1-y)B_(y)(XO₄)_(b)/M_(c)N_(d), wherein:

A includes one or more transition metals from the first row including without limitation Fe, Mn, Ni, V, Co and Ti;

B includes one or more doped metals including without limitation Fe, Mn, Ni, V, Co, Ti, Mg, Ca, Cu, Nb, Zr and other rare earth elements or metals;

X includes one or more members of P, Si, S, V and Ge;

M includes one or more metals selected from groups IA, IIA, IIIA, IVA, VA, IIIB, IVB and VB of the periodic table;

N includes one or more members of O, N, H, S, SO₄, PO₄, OH, Cl and fluorine-related elements; and

0≦a≦1, 0≦y≦0.5, 0<b≦1, 0<c≦4 and 0<d≦6.

The mixed crystal compound includes a lithium compound [Li_(a)A_(1-y)B_(y)(XO₄)_(b)] portion and a metal compound M_(c)N_(d) portion having a mixed crystalline relationship, with the lithium compound serving as the backbone or main building block of the cathode material. In one instance, the metal compound can be distributed into the lithium compound to provide a composite compound or a mixed crystal.

The cathode material may also include doped carbon additives, e.g., the mixed crystal compound Li_(a)A_(1-y)B_(y)(XO₄)_(b)/M_(c)N_(d) may be doped with carbon additives scattered between grain boundaries or coated on the grain surfaces. The doped carbon additive may provide the final cathode material product with 1-15% of carbon by weight. In one embodiment, the carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite and carbohydrate compound. In other embodiments, the carbon additive may include other carbon-related elements, precursors or compounds.

The microstructure of the mixed crystal compound, which is capable of being utilized as a cathode material, includes the lithium compound and the metal compound having a mixed-crystalline structure with mixed crystal lattices. The cathode material can come in at least three possible forms: smaller crystals residing within a larger crystal lattice, smaller crystal residing in between grain boundaries of large crystals, or smaller crystals residing on the exterior grain surfaces of a large crystal.

Reference is now made to FIGS. 1-4 illustrating a mixed crystal 10 having the chemical formula Li_(a)A_(1-y)B_(y)(XO₄)_(b)/M_(c)N_(d) according to an embodiment of the presently disclosed invention. Specifically, the mixed crystal 10 includes a mixture of a lithium compound [Li_(a)A_(1-y)B_(y)(XO₄)_(b)] 12 and a mixed metal crystal or metal compound [M_(c)N_(d)] 14. The lithium compound 12 has a larger crystal lattice while the metal compound 14 has a smaller crystal lattice.

In one instance, the metal compound 14, having a smaller crystal lattice 14, may be received or distributed within the lithium compound 12 having the larger crystal lattice 12 as best illustrated in FIG. 1. In another instance, the metal compound 14 can be received or distributed between two or more large crystal lattices 12 as best illustrated in FIG. 2. Alternatively, the metal compound 14 can reside within grain boundaries of the lithium compound 12 as best illustrated in FIG. 3. Lastly, the metal compound 14 may be dispersed about the exterior grain surfaces of the lithium compound 12 as best illustrated in FIG. 4. In all of these instances, lithium ion migration serves as a bridge either within a crystal lattice or in between two or more crystal lattices, wherein lithium ions can be fully released for enhanced electrical properties including electrical conductance, capacitance and recyclability. The mixed crystal may also provide enhanced electrochemical properties.

In other embodiments, the mixed crystal 10 may take on mixed crystalline forms. In other words, during formation of the metal compound 14 by mixing at least two metal oxides, a large number of crystal defects may be introduced within the intermediary or composite crystals such that the electronic states and formation of the metal oxides are altered or changed. The metal compound 14 with its mixed crystalline structure, therefore, contains a large number of oxygen vacancies and missing oxygen atoms. The oxygen vacancies can facilitate carrier conduction thereby enhancing the conductivity of the mixed crystal 10. The formation of the metal compound 14 having two or more metal oxides will become more apparent in subsequent discussion.

In some embodiments, the metal compound 14 can be received between the grain boundaries or on the exterior crystal lattices of the lithium compound 12 in forming the mixed crystal 10 as described above. In the alternative, the metal compound 14 and the lithium compound 12 may be heated or sintered at about 600-900° C. in an inert gas or reducing gas atmosphere for at least 2 hours. The resulting mixed crystal 10 provides an enhanced active material with improved electrical properties including conductivity and electrochemical properties thereby enhancing conductivity and charging capacity of a lithium secondary battery.

In one embodiment, the lithium compound has the general formula LiM_(a)N_(b)XO_(c), wherein: M is a first-row transition metal including Fe, Mn, Ni, V, Co and Ti; N is a metal selected from the group Fe, Mn, Ni, V, Co, Ti, Mg, Ca, Cu, Nb, Zr and rare-earth metals; X is selected from elements P, Si, S, V and Ge; and a, b and c have respective values that would render said lithium compound charge-neutral. The lithium compound can include a metal intercalation compound having a similar general formula. In other embodiments, the lithium compound has the general formula Li_(a)A_(1-y)B_(y)(XO₄)_(b), wherein: A is a first-row transition metal including Fe, Mn, Ni, V, Co and Ti; B is a metal selected from the group Fe, Mn, Ni, V, Co, Ti, Mg, Ca, Cu, Nb, Zr and rare-earth metals; X is selected from elements P, Si, S, V and Ge; and 0<a≦1, 0≦y≦0.5 and 0<b≦1. In yet another embodiment, the metal compound has the general formula M_(c)N_(d), wherein M is metal selected from IA, IIA, IIIA, IVA, VA, IIIB, IVB and VB groups in the periodic table; N is selected from O, N, H, S, SO₄, PO₄, OH, Cl, F, and C; and 0<c≦4 and 0<d≦6.

In another embodiment, a cathode material for lithium secondary batteries can be provided by sintering lithium iron phosphate (LiFePO₄) with a mixture compound, the cathode material capable of providing LiFePO₄:mixture compound molar ratios of 1:0.001-0.1. In this embodiment, the mixture compound can be formed of two or more metal oxides wherein the metal can be selected from groups IA, IIA, IIIA, IVA, VA, IIIB, IVB and VB of the periodic table. In another embodiment, the weight of a first metal oxide is about 0.5-20% of the weight of a second metal oxide.

The mixture of metal oxides can take on a mixed crystal configuration. Based on mixed crystal formation theory, mixing of two or more metal oxides can form a composite mixed metal crystal such that a plurality of crystal defects are introduced to the crystal structure and lattice. The electronic states of the metal oxides are altered or changed thereby producing a large number of oxygen atom vacancies. These vacancies facilitate electronic carrier conductions thus producing a highly conductive mixed metal crystal.

The metal mixture compound, having a mixed crystalline configuration, can be coupled to the crystal lattices of LiFePO₄ by a heating or sintering process. Alternatively, after the metal oxides have been heated and the mixed metal crystal has been formed, the mixed metal crystal can be coupled to the crystal lattices of LiFePO₄ to provide a lithium iron phosphate cathode material with a mixed crystal structure and configuration. The resulting mixed crystal structure can effectively improve the conductivity, electrochemical properties, and greatly enhance the charge capacity of the lithium secondary battery.

In other embodiments, the lithium iron phosphate cathode material can further include carbon coating on the exterior surfaces of the sintered product, the amount of carbon material added being capable of providing the final product with 1-15% of carbon by weight. The types of carbon material that can be utilized include without limitation one or more of carbon black, acetylene black, graphite and carbohydrate compound.

The invention also includes batteries made from the new cathode materials described in other embodiments.

A method of preparing a mixed crystal lithium iron phosphate cathode material includes evenly mixing at least one LiFePO₄ compound with a mixture compound and heating the resulting mixture to 600-900° C. in an inert gas or reducing gas atmosphere for between 2-48 hours. The mixture compound includes two or more metal oxides wherein the metal can be selected from groups IA, IIA, IIIA, IVA, VA, IIIB, IVB and VB of the periodic table. The mixture compound provides a mixed crystalline structure, wherein a method of preparing the mixture compound with the corresponding mixed crystalline structure includes mixing metal oxides from groups IA, IIA, IIIA, IVA, VA, IIIB, IVB and VB, and heating the mixture to 600-1200° C. for between 2-48 hours.

The LiFePO₄ compound may be prepared by providing lithium, iron and phosphate sources to provide Li, Fe and P atoms with Li:Fe:P molar ratios of 1:1:1. In other embodiments, different Li:Fe:P molar ratios may be adopted. The mixture can accordingly be grinded in a ball mill for 2-48 hours, dried between 40-80° C. or stirred until dry, and heated to 600-900° C. in an inert gas or reducing gas atmosphere for between 2-48 hours.

After combining the LiFePO₄ compound with the mixture compound having mixed crystalline structure, carbon additives can be provided to the resulting mixture and sintered to facilitate carbon coating. The amount of carbon additives is capable of providing the resulting lithium iron phosphate cathode material with 1-15% of carbon by weight. The types of carbon material that can be utilized include without limitation one or more of carbon black, acetylene black, graphite and carbohydrate compound. The carbon coating process further enhances the electrical conductivity of the cathode material.

Another method of preparing a mixed crystal cathode material includes evenly mixing lithium, iron and phosphate sources and heat to 600-900° C. in an inert gas or reducing gas atmosphere for at least 2 hours. The resulting mixture can then be combined with a mixed metal compound having a combination of two or more metal oxides selected from groups IA, IIA, IIIA, IVA, VA, IIIB, IVB and VB of the periodic table. In one embodiment, the lithium source, iron source, phosphate source and mixed metal compound are capable of providing Li:Fe:P:mixed metal compound molar ratios of 1:1:1:0.001-0.1. In other embodiments, different Li:Fe:P:mixed metal compound molar ratios may be adopted. Furthermore, at least one carbon source can be added to the resulting mixture, the carbon source including one or more of the following without limitation: carbon black, acetylene black, graphite and carbohydrate compound. The amount of carbon source added to the resulting mixture should be able to provide the final product with 1-15% of carbon by weight.

According to the presently disclosed embodiments, lithium sources capable of being used in preparing the cathode material include one or more of the following compounds without limitation: lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, lithium fluoride, lithium chloride, lithium bromide, lithium iodide and lithium dihydrogen phosphate. Likewise, iron sources include one or more of the following compounds without limitation: ferrous oxalate, ferrous acetate, ferrous chloride, ferrous sulfate, iron phosphate, ferrous oxide, ferric oxide, iron oxide and ferric phosphate. When using a trivalent iron compound as a source of iron, the ball milling process requires adding a carbon source to reduce the trivalent iron to a divalent iron. Furthermore, phosphorous sources include one or more of the following compounds without limitation: ammonium, ammonium phosphate, ammonium dihydrogen phosphate, iron phosphate, ferric phosphate and lithium hydrogen phosphate.

During the mixing process, specifically grinding in a ball mill, one or more solvents may be introduced including ethanol, DI water and acetone. In other embodiments, other mixing media and solvents may be utilized. In addition, the mixture can be dried between 40-80° C. or stirred until dry.

The types of inert gases that may be utilized include helium, neon, argon, krypton, xenon, radon and nitrogen. Additionally, reducing gases including hydrogen and carbon monoxide can also be incorporated. Other suitable gases may also be adopted.

It is understood that other lithium, iron, phosphorous and carbon sources may be utilized along with suitable solvents, inert gases and reducing gases as will be appreciated by one skilled in the art.

It is understood that the new cathode materials described above can be used to make lithium secondary batteries and other types of batteries.

The following are various embodiments of the mixed-crystal lithium iron phosphate cathode materials according to the presently disclosed invention.

EXAMPLE 1

Mix LiFePO₄ with [Y₂O₃ and Sb₂O₃ (mass ratio 0.2:1)] to provide [LiFePO₄:(Y₂O₃ and Sb₂O₃)] molar ratio of [1:(0.04)], add carbon-containing acetylene black (amount of carbon capable of providing 10% by weight of carbon content in the final product), grind the mixture in a ball mill for 15 hours, remove and dry at 60° C. Heat the resulting powder in a nitrogen atmosphere at 650° C. for 5 hours to provide a LiFePO₄ composite cathode material.

EXAMPLE 2

Mix Sb₂O₃ and TiO₂ (mass ratio 0.15:1), grind the mixture in a ball mill for 5 hours, remove and dry at 60° C. Heat the resulting powder at 1000° C. for 8 hours to provide a Sb₂O₃ and TiO₂ mixed compound. Under x-ray diffraction (XRD), the mixed compound did not exhibit new characteristic peaks on the XRD pattern indicating that the two oxides did not generate a new oxide compound. See FIGS. 5 & 6. The mixed compound, therefore, remained in a mixed crystal state indicative of a mixed crystal structure.

Mix LiFePO₄ with the mixed crystal to provide a molar ratio of 1 to 0.02, add carbon-containing glucose (amount of carbon capable of providing 8% by weight of carbon content in the final product), grind the mixture in a ball mill for 20 hours, remove and dry at 60° C. Heat the resulting powder in a nitrogen atmosphere at 750° C. for 8 hours to provide a LiFePO₄ composite cathode material.

EXAMPLE 3

Mix lithium fluoride, iron phosphate and diammonium phosphate to provide Li:Fe:P atomic ratio of 1.02:1:1, grind the mixture in a ball mill for 20 hours, remove and dry at 65° C. Heat the resulting powder in a nitrogen atmosphere at 750° C. for 12 hours to provide LiFePO₄.

Mix V₂O₅ and TiO₂ (mass ratio 0.08:1), grind the mixture in a ball mill for 8 hours, remove and dry at 65° C. Heat the resulting powder at 500° C. for 8 hours to provide a V₂O₅ and TiO₂ mixed compound. Under x-ray diffraction (XRD), the mixed compound did not exhibit new characteristic peaks on the XRD pattern indicating that the two oxides did not generate a new oxide compound. The mixed compound, therefore, remained in a mixed crystal state indicative of a mixed crystal structure.

Mix LiFePO₄ with the mixed crystal to provide a molar ratio of 1 to 0.05, grind the mixture in a ball mill for 10 hours, remove and dry at 60° C. Heat the resulting powder in a nitrogen atmosphere at 750° C. for 8 hours to provide a LiFePO₄ composite cathode material.

EXAMPLE 4

Mix MgO and Al₂O₃ (mass ratio 0.05:1), grind the mixture in a ball mill for 6 hours, remove and dry at 60° C. Heat the resulting powder to 1000° C. for 6 hours to provide a MgO and Al₂O₃ mixed compound. Under x-ray diffraction (XRD), the mixed compound did not exhibit new characteristic peaks on the XRD pattern indicating that the two oxides did not generate a new oxide compound. The mixed compound, therefore, remained in a mixed crystal state indicative of a mixed crystal structure.

Mix LiFePO₄ with the mixed crystal to provide a molar ratio of 1 to 0.002, add carbon-containing graphite (amount of carbon capable of providing 15% by weight of carbon content in the final product), grind the mixture in a ball mill for 15 hours, remove and dry at 65° C. Heat the resulting powder in a nitrogen atmosphere at 700° C. for 10 hours to provide a LiFePO₄ composite cathode material.

EXAMPLE 5

Mix lithium carbonate, ferric oxide, diammonium phosphate, SnO₂ and Nb₂O₅ to provide Li:Fe:P:(SnO₂ and Nb₂O₅) molar ratio of 1.01:1:1:0.04, wherein SnO₂ is 5% of Nb₂O₅ by mass and may be added at the same time to bring about reduction of the ferric oxide along with carbon-containing acetylene black (amount of carbon capable of providing 5% by weight of carbon content in the final product), grind the mixture in a ball mill for 24 hours, and stir at 65° C. until dry. Heat the resulting powder in a nitrogen atmosphere at 750° C. for 20 hours to provide a LiFePO₄ composite cathode material.

EXAMPLE 6

Mix lithium carbonate, ferrous oxalate, diammonium phosphate, SnO₂ and TiO₂ to provide Li:Fe:P:(SnO₂ and TiO₂) molar ratio of 1.02:1:1:0.03, wherein SnO₂ is 15% of TiO₂ by mass, carbon-containing sucrose (amount of carbon capable of providing 7% by weight of carbon content in the final product), grind the mixture in a ball mill for 20 hours, remove and dry at 65° C. Heat the resulting powder in a nitrogen atmosphere at 750° C. for 18 hours to provide a LiFePO₄ composite cathode material.

EXAMPLE 7

Mix lithium carbonate, ferrous phosphate, Nb₂O₅ and TiO₂ to provide Li:Fe:P:(Nb₂O₅ and TiO₂) molar ratio of 1:1:1:0.01, wherein Nb₂O₅ is 5% of TiO₂ by mass, carbon-containing sucrose (amount of carbon capable of providing 5% by weight of carbon content in the final product), grind the mixture in a ball mill for 20 hours, remove and dry at 65° C. Heat the resulting powder in a nitrogen atmosphere at 750° C. for 15 hours to provide a LiFePO₄ composite cathode material.

Reference 1

Mix lithium carbonate, ferrous oxalate, copper fluoride and diammonium phosphate to provide a molar ratio of 1:0.9:0.1:1, add carbon-containing sucrose (amount of carbon capable of providing 7% by weight of carbon content in the final product), grind the mixture in a ball mill for 10 hours, remove and dry at 70° C. Heat the resulting powder in a nitrogen atmosphere at 650° C. for 20 hours to provide a LiFePO₄ composite cathode material.

TESTING OF EXAMPLES 1-7 AND REFERENCE 1

(1) Battery Preparation

(a) Cathode Active Material

Separately combine 100 grams of each of the LiFePO₄ composite material from examples 1-6 and reference 1 with 3 grams of polyvinylidene fluoride (PVDF) binder and 2 grams of acetylene black to 50 grams of N-methylpyrrolidone (NMP), mix in a vacuum mixer into a uniform slurry, apply a coating of about 20 microns thick to each side of an aluminum foil, dry at 150° C., roll and crop to a size of 480×44 mm² to provide about 2.8 grams of cathode active material.

(b) Anode Active Material

Combine 100 grams of natural graphite with 3 grams of polyvinylidene fluoride (PVDF) binder and 3 grams of conductive acetylene black to 100 grams of N-methylpyrrolidone (NMP), mix in a vacuum mixer into a uniform slurry, apply a coating of about 12 microns thick to each side of a copper foil, dry at 90° C., roll and crop to a size of 485×45 mm² to provide about 2.6 grams of anode active material.

(c) Battery Assembly

Separately wind each of the cathode and anode active materials with polypropylene film into a lithium secondary battery core, followed by dissolving one mole of LiPF₆ in a mixture of non-aqueous electrolyte solvent EC/EMC/DEC to provide a ratio of 1:1:1, inject and seal the electrolyte having a capacity of 3.8 g/Ah into the battery to provide separate lithium secondary batteries for testing.

(2) Specific Discharge Capacity Test

Using a current charge of 0.2 C, charge each battery for 4 hours, and then at constant voltage to 3.8 V. After setting the battery aside for 20 minutes, using a current of 0.2 C discharge from 3.8 V to 3.0 V, record the battery's initial discharge capacity, and use the following equation to calculate the battery's initial specific capacity:

Initial specific capacity=Initial discharge capacity (milliampere hour)/weight of cathode active material (grams).

(3) Measure the Specific Capacity After 500 Cycles

(4) Separately Measure Specific Capacity at 1 C, 3 C and 5 C

The testing cycle results for examples 1-7 and reference 1 are shown in Table 1.

TABLE 1 Test results of LiFePO₄ composite cathode materials and reference sample. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ref. 1 Initial specific capacity (mAh/g) 130 125 126 125 128 131 131 98 Specific capacity after 500 cycles (mAh/g) 128 124 124 123 126 130 128 62 Specific capacity at 1 C (mAh/g) 126 120 120 121 122 126 124 80 Specific capacity at 3 C (mAh/g) 111 107 107 106 109 112 116 50 Specific capacity at 5 C (mAh/g) 108 105 106 105 106 109 109 34

From the data in Table 1, it can be observed that the LiFePO₄ composite cathode materials according to examples 1-7 of the presently disclosed invention provide higher initial specific discharge capacity than reference 1. Accordingly, the LiFePO₄ composite cathode materials for lithium secondary batteries and methods of manufacturing such according to the presently disclosed embodiments provide superior electrical performance, e.g., higher discharge capacity, low capacity loss after multiple cycles, and high discharge capacity retention rate.

Additional experimental data are provided in Tables 2 and 3 illustrating the electrical properties of some of the examples described above.

TABLE 2 Middle Specific Charging Discharging discharge capacity capacity capacity Efficiency voltage Set capacity (mAh/g) Example 1 11.16 9.91 88.8% 3.372 9.60 154.86 Example 3 10.89 10.02 91.9% 3.371 9.74 156.48 Example 4 11.22 10.18 90.7% 3.376 9.88 159.09 Example 5 10.84 10.01 92.3% 3.375 9.74 156.34 Minimum 10.84 9.91 88.8% 3.37 9.60 154.86 Average 11.03 10.03 90.9% 3.37 9.74 156.70 Maximum 11.22 10.18 92.3% 3.38 9.88 159.09 Range 0.38 0.27 3.5% 0.00 0.28 4.23 Median 11.03 10.01 91.3% 3.37 9.74 156.41 STDEV 0.19 0.11 1.6% 0.00 0.12 1.76

TABLE 3 Middle Specific Charging Discharging discharge capacity capacity capacity Efficiency voltage Set capacity (mAh/g) Example 1 11.33 10.20 90.0% 3.343 9.67 159.38 Example 2 10.16 9.19 90.5% 3.362 8.75 143.66 Example 4 11.19 10.18 90.9% 3.367 9.67 159.06 Example 5 11.21 10.20 91.0% 3.350 9.67 159.36 Minimum 10.16 9.19 90.0% 3.34 8.75 143.66 Average 10.97 9.94 90.6% 3.36 9.44 155.36 Maximum 11.33 10.20 91.0% 3.37 9.67 159.38 Range 1.17 1.01 1.0% 0.02 0.92 15.72 Median 11.20 10.19 90.7% 3.36 9.67 159.21 STDEV 0.55 0.50 0.4% 0.01 0.46 7.81

Reference is now made to FIG. 5 illustrating an x-ray diffraction (XRD) pattern of a cathode composite compound according to example 7. As shown in the figure, the TiO₂ peak from the starting material is missing after formation of the cathode composite compound, which is suggestive of a substance having mixed crystalline structure or a mixed crystal compound. It is also possible that the TiO₂ exchange has taken place with Fe atoms. However, it is suspected that a mixed crystal is the most likely outcome.

In another embodiment, a cathode active material can be provided having a mixed crystal structure, the mixed crystal structure having a first crystalline substance having one or more members with the general formulas Li_(x)M_(y)(XO₄)_(z), LiMXO₅, LiMXO₆ and LiMX₂O₇, wherein:

0<x/z≦1 and 0≦y/z≦1;

M is selected from elements Na, Mn, Fe, Co, Ni, Ti, Nb and V; and

X is selected from elements P, S, As, Mo and W.

The mixed crystal structure further includes a second crystalline substance having one or more members with the general formula A_(a)M_(b)N_(c)O_(d), wherein:

A, M and N are different metals selected from groups IIA, IIIA, IVA, VA, IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table;

0≦a≦6, 0≦b≦6, 0<c≦6 and 0<d≦12; and

wherein a and b cannot both be 0 at the same time.

The mixed crystal structure can be formed by sintering two or more compounds, the intermediary mixture having oxygen vacancies or metallic crystalline structures. The two or more compounds do not exhibit any major chemical reactions when mixed together. However, upon sintering, a large number of crystalline defects can be formed thereby altering the electronic states of the compounds creating a large number of oxygen vacancies. The sintering process can be carried out by heating the compounds at a rate of about 5 to 20° C. per minute to temperatures of about 500 to 850° C. for about 5 to 32 hours. These oxygen vacancies provide the needed carriers thus greatly enhancing the electrical conductivity of the mixed crystal. Accordingly, the cathode active material can achieve electrical conductivity of about 0.01 to 10 S/cm (Siemens per centimeter) at about 25° C., the electrical conductivity values being greater than traditional lithium iron phosphate cathode active materials. In this embodiment, the first crystalline substance and the second crystalline substance have molar ratios of about 1 to 0.01-0.05.

The first crystalline substance can have a mixed crystalline structure with the general formula Li_(x)M_(y)(XO₄)_(z) including one or more members selected from the group consisting of LiFePO₄, LiMnPO₄ and LiCoPO₄. In other embodiments, single-crystalline structures including Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃ and Li₂NaV₂(PO₄)₃ may be incorporated. For the general formula LiMXO₅, the first crystalline substance can be LiTiPO₅. For the general formula LiMXO₆, the first crystalline substance can include LiVMoO₆ and LiVWO₆. For the general formula LiMX₂O₇, the first crystalline substance can include LiVP₂O₇ and LiFeAs₂O₇. The second crystalline substance can include one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆.

In one embodiment, the second crystalline substance can be formed by heating oxide compounds of A, M and N at molar ratio of a:b:c at about 400 to 1000° C. for about 8 to 15 hours. The sintered product can be measured with a Rigaku D/MAX-2200/PC x-ray diffraction (XRD) instrument to acquire an XRD pattern, which can subsequently be compared with known chemical standards provided by the system. The oxide compound or oxygen-containing compound of A can be oxides of A and/or sintered products of oxides of A and other compounds, the oxides of A and other compounds include hydroxides of A, carbonates of A, and bicarbonates of A. Likewise, the oxide compound or oxygen-containing compound of M can be oxides of M and/or sintered products of oxides of M and other compounds, the oxides of M and other compounds include hydroxides of M, carbonates of M, and bicarbonates of M. Similarly, the oxide compound or oxygen-containing compound of N can be oxides of N and/or sintered products of oxides of N and other compounds, the oxides of N and other compounds include hydroxides of N, carbonates of N, and bicarbonates of N.

The cathode active material can further include a carbon additive, wherein the carbon additive is capable of providing the mixed crystal structure with about 1-5% of carbon by weight. The carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin, polyethylene glycol, and other organic and inorganic sources.

In another embodiment, a cathode active material can be provided, the cathode active material having a mixed crystal, the mixed crystal having: a first crystalline substance having a combination of lithium, iron and phosphorous sources, wherein the lithium, iron and phosphorous (Li:Fe:P) sources have molar ratios of about 0.95-1.1:1:0.95-1.1; and a second crystalline substance having one or more members with the general formula A_(a)M_(b)N_(c)O_(d), wherein:

A, M and N are different metals selected from groups IIA, IIIA, IVA, VA, IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table;

0≦a≦6, 0≦b≦6, 0<c≦6 and 0<d≦12; and

wherein a and b cannot both be 0 at the same time.

Like above, the mixed crystal structure in this embodiment can be formed by sintering two or more compounds with the intermediary mixture having oxygen vacancies or metallic crystalline structures for providing the carriers thus enhancing the electrical conductivity of the mixed crystal. The sintering process can be carried out by heating the compounds at a rate of about 5 to 20° C. per minute to temperatures of about 500 to 850° C. for about 5 to 32 hours whereby the cathode active material can achieve electrical conductivity of about 0.01 to 10 S/cm (Siemens per centimeter) at about 25° C. In this embodiment, the phosphorous source and the second crystalline substance has molar ratios of about 1 to 0.01-0.05, taking only the phosphorous source in the material into consideration. Further, the second crystalline substance can be formed by heating oxide compounds of A, M and N at molar ratio of a:b:c at about 400 to 1000° C. for about 8 to 15 hours. The oxide compounds of A, M and N being similar to those described above.

The second crystalline substance includes one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆.

The lithium source includes one or more members selected from the group consisting of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate; the iron source includes one or more members selected from the group consisting of ferrous oxalate, ferrous carbonate, iron acetate, iron oxide, iron phosphate, iron pyrophosphate and iron nitrate; and the phosphate source includes one or more members selected from the group consisting of ammonium phosphate, ammonium dihydrogen phosphate, ammonium, iron phosphate, phosphoric acid and lithium dihydrogen phosphate.

The cathode active material further includes a carbon additive, wherein the carbon additive is capable of providing the mixed crystal with about 1-5% of carbon by weight. The carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol, and other organic and inorganic sources.

In another embodiment, a cathode active material can be provided, the cathode active material having a mixed crystal, the mixed crystal having: a first crystalline substance having one or more members selected from the group consisting of LiFePO₄, LiMnPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃, Li₂NaV₂(PO₄)₃, LiTiPO₅, LiVMoO₆, LiVWO₆, LiVP₂O₇ and LiFeAs₂O₇; and a second crystalline substance having one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTi₄ and MgNb₂O₆.

The cathode active material can achieve electrical conductivity of about 0.01 to 10 S/cm at about 25° C. The cathode active material further includes a carbon additive, wherein the carbon additive is capable of providing the mixed crystal with about 1-5% of carbon by weight. The carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol, and other organic and inorganic sources.

In other embodiments, a lithium ion secondary battery can be provided, the lithium ion secondary battery having a battery case, electrodes and electrolyte, the electrodes and electrolyte being sealed within the battery case, the electrodes having wounded or stacked cathode, anode and divider film, the cathode further including the cathode active materials described above.

The cathode includes cathode components such as the cathode active materials disclosed in the embodiments above along with adhesives. The adhesives can be hydrophobic or hydrophilic binding additives without any specific binder ratio restrictions. In one instance, the hydrophilic to hydrophobic adhesive binder can have weight ratios of about 0.3:1 to about 1:1. The adhesive can solid, aqueous or as an emulsion. The concentration can be adjusted accordingly based on methods of preparing the cathode, anode and the slurry viscosity and coating. In one example, the hydrophilic adhesive solution has a concentration of about 0.5 to 4 weight percent while the hydrophobic latex binder has a concentration of about 10 to 80 weight percent.

Hydrophobic adhesives can include PTFE, styrene butadiene rubber, or mixtures thereof. Hydrophilic adhesives can include HPMC, CMC, hydroxyethyl cellulose, polyvinyl alcohol, or mixtures thereof. The binder content can be about 0.01 to 8% by weight of the total cathode active material.

In addition, conductive agents may be incorporated in the cathode active material, the conductive agents include without limitation graphite, carbon fiber, carbon black, metal powders and fibers as well as any suitable material understood by one skilled in the art. The conductive agent can be about 0.1 to 20% by weight of the total cathode active material.

The method of preparing the cathode include using solvents to dissolve the cathode active material and mixing with adhesives and conductive agents to form a cathode slurry. The cathode slurry can be applied onto cathode collectors, dried, rolled or compressed, and sliced into pieces to produce the cathode. In one example, the slurry can be dried at about 100 to 150° C. for about 2 to 10 hours. The cathode collectors include aluminum foil, copper foil, nickel-plated steel or punched stainless steel. The types of solvent to use include N-methyl pyrrolidone (NMP), dimethylformamide (DMF), diethyl formamide (DEF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), water, alcohol and mixtures thereof. The amount of solvent to use can be adjusted accordingly to provide the proper slurry coating and viscosity. In one instance, the amount of solvent can be about 40 to 90% by weight of the cathode active material. The method of preparing the cathode and types of solvents, adhesives, conductive agents and cathode collectors can also incorporate other techniques understood by one skilled in the art.

As discussed above, the lithium secondary battery includes a battery shell, electrodes and electrolyte, the electrodes and electrolyte capable of being sealed within the battery shell. The electrodes include wounded or stacked cathode, anode and divider film with the cathode utilizing the cathode active material of the presently disclosed embodiments.

The divider film can be situated between the cathode and anode for preventing electrical shorts and for maintaining the electrolytic solution. In one instance, the divider film can include any membrane including without limitation micro-porous membrane polyolefin, polyethylene fibers, ultra-fine glass fibers and fiber paper.

The anode can incorporate any anode active materials and known methods of forming such materials as known in the arts. The anode active material can be provided in slurry form and coated onto anode collectors similar to the cathode collectors above. Additionally, the anode active material may include carbon additives such as non-carbon graphite, graphite, and polymers having undergone high-temperature carbon oxidation. The carbon additive can also include pyrolytic coal, coke, organic polymer sintered materials and activated carbons. The organic polymer sintered materials include phenolic resin, epoxy resin, and carbonized products obtained by sintering.

Adhesives can utilize traditional adhesives for lithium secondary batteries including polyvinyl alcohol, PTFE, carboxymethyl cellulose (CMC), hydroxymethyl cellulose (HMC), and styrene butadiene rubber (SBR). The adhesive binder can be about 0.5 to 8 weight percent of the total anode active material.

The anode active material can further include conductive agents, the conductive agent capable of increasing electrical conductivity and reducing internal resistance of the battery. The conductive agent includes carbon black, nickel powder and copper powder. Other conductive agents known by one skilled in the art may also be utilized and can be about 0.1 to 12 weight percent of the anode active material.

The method of preparing the anode includes using solvents to dissolve the anode active material and mixing with adhesives and conductive agents to form an anode slurry. The anode slurry can be applied onto the anode collectors similar to that of the cathode slurry described above, dried, rolled or compressed, and sliced into pieces to produce the anode. In one example, the slurry can be dried at about 100 to 150° C. for about 2 to 10 hours. The types of solvent for dissolving the anode active material include N-methyl pyrrolidone (NMP), dimethylformamide (DMF), diethyl formamide (DEF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), water, alcohol and mixtures thereof. The amount and concentration of solvents to use can be adjusted accordingly to provide the proper slurry coating and viscosity. Like the cathode slurry, the amount of anode slurry applied to the anode collector can be about 40 to 90 weight percent of the anode active material.

The electrolyte for the lithium secondary battery can be a non-aqueous electrolyte, which can be formed by dissolving lithium salt in a non-aqueous solvent. The lithium salt electrolyte can include one or more members selected from lithium hexafluorophosphate (LiPF₆), lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄), lithium hexafluoroarsenate (LiAsF₆), lithium hexafluorosilicate (LiSiF₆), lithium tetraphenylborate (LiB(C₆H₅)₄), lithium chloride (LiCl), lithium bromide (LiBr), lithium aluminum tetrachloride (LiAlCl₄), LiC(SO₂CF₃)₃, LiCH₃SO₃, and LiN(SO₂CF₃)₂. The aqueous solvent can be chain ester and ester ring mixed solution, the chain ester being one or more members of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), dimethylpropyl carbonate (DPC) and other fluoride or sulfur-containing unsaturated key chain organic esters, with the ester ring being one or more members of ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), gamma-butyrolactone (γ-BL), sodium fluoride and other lactone-containing or unsaturated organic ester rings. In one instance, the lithium salt electrolyte has a concentration of about 0.1 to 2 mole per liter.

The presently disclosed lithium secondary batteries can be provided by processes known by one skilled in the art. The preparation method includes winding or stacking cathode, anode and divider films into the battery core, and placing the battery core into the battery shell, adding the electrolyte, and seal the battery accordingly. The winding, stacking and sealing of the batteries can utilize traditional techniques as understood by one skilled in the art. Furthermore, other known steps of manufacturing the lithium secondary battery can be incorporated.

The following are various embodiments of mixed-crystal cathode active materials according to the presently disclosed invention.

EXAMPLE A1

Mix MnCO₃, TiO₂ and Nb₂O₅ in a molar ratio of 1:1:1, grind the mixture in a ball mill for 5 hours, heat in a nitrogen atmosphere at 10° C. per minute to 500° C. and continue sintering the product for 10 hours. Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the resulting product. In comparison with the standard XRD pattern for MnTiNb₂O₈, it was determined that the sintered product is MnTiNb₂O₈.

Mix LiFePO₄ with the resulting MnTiNb₂O₈ from above in a molar ratio of 1:0.04, add starch as a source of carbon (amount of carbon capable of providing 5% by weight of carbon content in the final product). In one example, the LiFePO₄ can be prepared by mixing lithium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate in a Li:Fe:P molar ratio of 1:1:1. Alternatively, the LiFePO₄ can be prepared by other lithium, iron and phosphate sources or by a third party.

Grind the mixture in a ball mill for 10 hours, remove and dry at 80° C. Heat the resulting powder in a nitrogen or argon atmosphere at 10° C. per minute to 600° C., continue sintering the product for 20 hours to provide a LiFePO₄/MnTiNb₂O₈/C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the cathode active material as shown in FIG. 6. Looking at the diffraction peaks of the sintered product, other than peaks corresponding to LiFePO₄ and MnTiNb₂O₈, there are no new peaks or features which is an indication that the LiFePO₄ and MnTiNb₂O₈ exist in two phases and that no new compounds are created. Accordingly, this demonstrates that the process described above provides a cathode active material having LiFePO₄/MnTiNb₂O₈/C in a mixed crystal form.

EXAMPLE A2

Mix ZnO and Ta₂O₅ in a molar ratio of 1:1, grind the mixture in a ball mill for 5 hours, heat in an oxygen atmosphere at 15° C. per minute to 800° C. and continue sintering the product for 8 hours. Using the Rigaku D/MAX-2200/PC, an XRD pattern can be carried out on the resulting product. In comparison with the standard XRD pattern for ZnTa₂O₆, it was determined that the sintered product is ZnTa₂O₆.

Mix LiFePO₄ with the resulting ZnTa₂O₆ from above in a molar ratio of 1:0.03 with acetylene black as a source of carbon (amount of carbon capable of providing 0% by weight of carbon content in the final product). The LiFePO₄ can be prepared by mixing lithium oxalate, iron oxide and diammonium hydrogen phosphate in a Li:Fe:P molar ratio of 0.95:1:1, which can be added to the ZnTa₂O₆ at a diammonium hydrogen phosphate to ZnTa₂O₆ molar ratio of 1:0.03 (taking into account the phosphorous components in the mixture). Alternatively, the LiFePO₄ can be prepared by other lithium, iron and phosphate sources or by a third party.

Grind the mixture in a ball mill for 10 hours, remove and dry at 80° C. Heat the resulting powder in a nitrogen or argon atmosphere at 5° C. per minute to 500° C., continue sintering the product for 30 hours to provide a LiFePO₄/ZnTa₂O₆ mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the cathode active material as shown in FIG. 7. Looking at the diffraction peaks of the sintered product, other than peaks corresponding to LiFePO₄ and ZnTa₂O₆, there are no new peaks or features which is an indication that the LiFePO₄ and ZnTa₂O₆ exist in two phases and that no new compounds are created. Accordingly, this demonstrates that the process described above provides a cathode active material having LiFePO₄/ZnTa₂O₆ in a mixed crystal form.

EXAMPLE A3

Mix Y₂O₃, BaCO₃ and TiO₂ in a molar ratio of 0.5:3:2, grind the mixture in a ball mill for 5 hours, heat in a nitrogen atmosphere at 7° C. per minute to 1000° C. and continue sintering the product for 15 hours. Using the Rigaku D/MAX-2200/PC, an XRD pattern can be carried out on the resulting product. In comparison with the standard XRD pattern for YBa₃Ti₂O_(8.5), it was determined that the sintered product is YBa₃Ti₂O_(8.5).

Mix LiFePO₄ with the resulting YBa₃Ti₂O_(8.5) from above in a molar ratio of 1:0.02 with carbon black as a source of carbon (amount of carbon capable of providing 2% by weight of carbon content in the final product). The LiFePO₄ can be prepared by mixing lithium hydroxide, ferrous carbonate and phosphoric acid in a Li:Fe:P molar ratio of 1.05:1:1.05, which can be added to the YBa₃Ti₂O_(8.5) at a phosphoric acid to YBa₃Ti₂O_(8.5) molar ratio of 1:0.02 (taking into account the phosphorous components in the mixture). Alternatively, the LiFePO₄ can be prepared by other lithium, iron and phosphate sources or by a third party.

Grind the mixture in a ball mill for 10 hours, remove and dry at 80° C. Heat the resulting powder in a nitrogen or argon atmosphere at 20° C. per minute to 800° C., continue sintering the product for 8 hours to provide a LiFePO₄/YBa₃Ti₂O_(8.5)/C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the cathode active material as shown in FIG. 8. Looking at the diffraction peaks of the sintered product, other than peaks corresponding to LiFePO₄ and YBa₃Ti₂O_(8.5), there are no new peaks or features which is an indication that the LiFePO₄ and YBa₃Ti₂O_(8.5) exist in two phases and that no new compounds are created. Accordingly, this demonstrates that the process described above provides a cathode active material having LiFePO₄/YBa₃Ti₂O_(8.5)/C in a mixed crystal form.

EXAMPLE A4

Mix CuO, ZnO and Nb₂O₅ in a molar ratio of 0.85:0.15:1, grind the mixture in a ball mill for 5 hours, heat in a nitrogen atmosphere at 7° C. per minute to 1000° C. and continue sintering the product for 15 hours. Using the Rigaku D/MAX-2200/PC, an XRD pattern can be carried out on the resulting product. In comparison with the standard XRD pattern for Cu_(0.85)Zn_(0.15)Nb₂O₆, it was determined that the sintered product is Cu_(0.85)Zn_(0.15)Nb₂O₆.

The remaining steps incorporate those used in Example A1, with the difference being that the Cu_(0.85)Zn_(0.15)Nb₂O₆ is substituted in place of the MnTiNb₂O₈ to provide a LiFePO₄/Cu_(0.85)Zn_(0.15)Nb₂O₆/C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the cathode active material as shown in FIG. 9. Looking at the diffraction peaks of the sintered product, other than peaks corresponding to LiFePO₄ and Cu_(0.85)Zn_(0.15)Nb₂O₆, there are no new peaks or features which is an indication that the LiFePO₄ and Cu_(0.85)Zn_(0.15)Nb₂O₆ exist in two phases and that no new compounds are created. Accordingly, this demonstrates that the process described above provides a cathode active material having LiFePO₄/Cu_(0.85)Zn_(0.15)Nb₂O₆/C in a mixed crystal form.

EXAMPLE A5

Mix ZrO₂, TiO₂ and SnO₂ in a molar ratio of 0.75:0.75:0.5, grind the mixture in a ball mill for 5 hours, heat in a nitrogen atmosphere at 7° C. per minute to 1000° C. and continue sintering the product for 15 hours. Using the Rigaku D/MAX-2200/PC, an XRD pattern can be carried out on the resulting product. In comparison with the standard XRD pattern for Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, it was determined that the sintered product is Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄.

The remaining steps incorporate those used in Example A1, with the difference being that the Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄ is substituted in place of the MnTiNb₂O₈ to provide a LiFePO₄/Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄/C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the cathode active material as shown in FIG. 10. Looking at the diffraction peaks of the sintered product, other than peaks corresponding to LiFePO₄ and Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, there are no new peaks or features which is an indication that the LiFePO₄ and Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄ exist in two phases and that no new compounds are created. Accordingly, this demonstrates that the process described above provides a cathode active material having LiFePO₄/Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄/C in a mixed crystal form.

EXAMPLE A6

Mix FeO and WO₃ in a molar ratio of 1:1, grind the mixture in a ball mill for 5 hours, heat in an oxygen atmosphere at 15° C. per minute to 800° C. and continue sintering the product for 8 hours. Using the Rigaku D/MAX-2200/PC, an XRD pattern can be carried out on the resulting product. In comparison with the standard XRD pattern for FeWO₄, it was determined that the sintered product is FeWO₄.

The remaining steps incorporate those used in Example A1, with the difference being that the FeWO₄ is substituted in place of the MnTiNb₂O₈ to provide a LiFePO₄/FeWO₄/C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the cathode active material as shown in FIG. 11. Looking at the diffraction peaks of the sintered product, other than peaks corresponding to LiFePO₄ and FeWO₄, there are no new peaks or features which is an indication that the LiFePO₄ and FeWO₄ exist in two phases and that no new compounds are created. Accordingly, this demonstrates that the process described above provides a cathode active material having LiFePO₄/FeWO₄/C in a mixed crystal form.

EXAMPLE A7

Mix ZnO and Al₂O₃ in a molar ratio of 2:1, grind the mixture in a ball mill for 5 hours, heat in an oxygen atmosphere at 15° C. per minute to 800° C. and continue sintering the product for 8 hours. Using the Rigaku D/MAX-2200/PC, an XRD pattern can be carried out on the resulting product. In comparison with the standard XRD pattern for ZnAlO₂, it was determined that the sintered product is ZnAlO₂.

The remaining steps incorporate those used in Example A1, with the difference being that the ZnAlO₂ is substituted in place of the MnTiNb₂O₈ to provide a LiFePO₄/ZnAlO₂/C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern was carried out on the cathode active material. Upon examination of the diffraction peaks of the sintered product, other than peaks corresponding to LiFePO₄ and ZnAlO₂, there are no new peaks or features which is an indication that the LiFePO₄ and ZnAlO₂ exist in two phases and that no new compounds are created. Accordingly, this demonstrates that the process described above provides a cathode active material having LiFePO₄/ZnAlO₂/C in a mixed crystal form.

Reference R1

Example B of U.S. Patent Application No. 2007/0207385 to provide a cathode material with LiFePO₄ to ZnAlO₂ molar ratio of 1:0.04 and carbon additive to provide a total carbon content of 5% by weight in the final product. The ZnAlO₂ and carbon additive are coated on the exterior surfaces of the LiFePO₄ to provide a cathode active material having LiFePO₄/ZnAlO₂/C.

CONDUCTIVITY OF EXAMPLES A1-A7 AND REFERENCE R1

At 25° C., separately take each cathode active materials of Examples A1-A7 and Reference R1, apply 30 MPa of pressure to provide a cylinder. Measure the height (t), diameter (d) and resistance (R) of each cylinder. Use the following formula to calculate the electrical conductivity (σ) for each sample:

Electrical conductivity σ=4×l/(πR×d²)

The electrical conductivities of Examples A1-A7 and Reference R1 are shown in Table 4.

TABLE 4 Electrical conductivities values of various samples at 25° C.. Electrical conductivity of cathode Sample No. active materials at 25° C. (S/cm) Example A1 0.175 Example A2 0.21 Example A3 1.8 Example A4 1.2 Example A5 1.5 Example A6 0.8 Example A7 0.5 Reference R1 2.6 × 10⁻⁵

From Table 4, it can be observed that the cathode active materials of the present embodiments can achieve electrical conductivity up to 1.8 S/cm (Siemens per centimeter). In contrast, the cathode active material of Reference R1, obtained by known having ZnAlO2 and carbon additives coated on the surfaces of LiFePO4, achieved electrical conductivity of 2.6×10⁻⁶ S/cm while the cathode active material of Example A7, having similar composition to that of Reference R1 but provided by the presently disclosed method, achieved electrical conductivity of 0.5 S/cm, the latter being 19,000 times more electrically conductive.

TESTING OF EXAMPLES A1-A7 AND REFERENCE R1

(1) Battery Preparation

(a) Cathode Active Material

Separately combine 90 grams of each of the composite cathode material from Examples A1-A7 and Reference R1 with 5 grams of polyvinylidene fluoride (PVDF) binder and 5 grams of acetylene black to 50 grams of N-methylpyrrolidone (NMP). Place in a vacuum mixer to mix into a uniform slurry. Apply a coating of about 20 microns thick on both sides of an aluminum foil, dry at 150° C., roll and crop to a size of 540×43.5 mm² to provide about 5.2 grams of cathode active material.

(b) Anode Active Material

Combine 90 grams of natural graphite with 5 grams of polyvinylidene fluoride (PVDF) binder and 5 grams of conductive carbon black to 100 grams of N-methylpyrrolidone (NMP). Place in a vacuum mixer to mix into a uniform slurry. Apply a coating of about 12 microns thick to both sides of a copper foil, dry at 90° C., roll and crop to a size of 500×44 mm² to provide about 3.8 grams of anode active material.

(c) Battery Assembly

Separately wind each of the cathode and anode active materials with polypropylene film into a lithium secondary battery core, followed by dissolving one mole of LiPF₆ in a mixture of non-aqueous electrolyte solvent EC/EMC/DEC to provide a ratio of 1:1:1, inject and seal the electrolyte having a capacity of 3.8 g/Ah into the battery to provide separate lithium secondary batteries B1-B7 (Examples A1-A7) and BC1 (Reference R1) for testing.

Performance Testing of Batteries B1-B7 and BC1

Separately place each of batteries B1-B7 and BC1 on the testing cabinet. At 25° C., charge each battery at a current of 0.5 C with a voltage limit of 3.8 V and set the battery aside for 20 minutes. Using a current of 0.5 C, discharge the battery from 3.8 V to 2.5 V and record the discharge capacity as the battery's initial discharge capacity. Use the following equation to calculate the battery's specific discharge capacity. The test results for batteries B1-B7 and BC1 are shown in Table 5.

Specific discharge capacity=Initial discharge capacity (milliampere hour)/ weight of cathode active material (grams)

Repeat the process described above: charge the battery, set it aside, and discharge each battery for 500 cycles. Record the battery's discharge capacity and use the following equation to calculate the battery's ability to maintain discharge capacity after 500 cycles. The higher the maintenance rate, the better the performance of the battery in maintaining its discharge capacity. The test results for batteries B1-B7 and BC1 are shown in Table 5.

Capacity maintenance rate=(Discharge capacity after nth cycle/initial discharge capacity )×100%

TABLE 5 Electrical testing results for batteries B1-B7 and BC1. Specific Discharge Capacity Capacity Maintenance Rate Sample No. (mAh/g at 0.5 C) after 500 Cycles Example A1/Battery B1 123 95.21% Example A2/Battery B2 124 95.87% Example A3/Battery B3 135 98.87% Example A4/Battery B4 131 97.56% Example A5/Battery B5 133 98.50% Example A6/Battery B6 128 97.05% Example A7/Battery B7 126 96.44% Reference R1/Battery BC1 112 90.12%

From the data in Table 5, it can be observed that the cathode active materials according to Examples A1-A7 of the presently disclosed invention are able to achieve better electrical performance than Reference R1. Specifically, the cathode active materials of batteries B1-B7 are able to achieve specific discharge capacity of at least 123 mAh/g at 0.5 C and maintain greater than 95% discharge capacity after 500 cycles. Additionally, the cathode active material of Reference R1 achieved specific discharge capacity of 112 mAh/g and maintained 90.12% discharge capacity after 500 cycles while cathode active material of Example A7, having similar composition to that of Reference R1 but provided by the presently disclosed method, achieved specific discharge capacity of 126 mAh/g and maintained 96.44% discharge capacity after 500 cycles. Accordingly, the cathode active materials for lithium secondary batteries and methods of manufacturing such according to the presently disclosed embodiments are able to provide superior electrical performance, e.g., higher electrical conductivity, discharge capacity and discharge capacity maintenance or retention rate.

Although the invention has been described in detail with reference to several embodiments, additional variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims. 

1. A method of preparing a cathode active material for lithium secondary batteries comprising: providing a first crystalline substance having one or more members with the general formulas Li_(x)M_(y)(XO₄)_(z), LiMXO₅, LiMXO₆ and LiMX₂O₇, wherein: 0<x/z≦1 and 0≦y/z≦1; M is selected from elements Na, Mn, Fe, Co, Ni, Ti, Nb and V; X is selected from elements P, S, As, Mo and W; providing a second crystalline substance having one or more members with the general formula A_(a)M_(b)N_(c)O_(d), wherein: A, M and N are different metals selected from groups IIA, IIIA, IVA, VA, IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table; 0≦a≦6, 0≦b≦6, 0<c≦6 and 0<d≦12; wherein a and b cannot both be 0 at the same time; and sintering the two crystalline substances at about 500 to 800° C. for about 5 to 32 hours to provide a mixed crystal, the mixed crystal having electrical conductivity of about 0.01 to 10 S/cm at about 25° C.
 2. The method of claim 1, wherein the first crystalline substance and the second crystalline substance has molar ratios of about 1 to 0.01-0.05.
 3. The method of claim 1, further comprising sintering a carbon additive with the two crystalline substances, the carbon additive capable of providing the mixed crystal with about 1-5% of carbon by weight.
 4. The method of claim 3, wherein the carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol.
 5. The method of claim 1, wherein the first crystalline substance includes one or more members selected from the group consisting of LiFePO₄, LiMnPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃, Li₂NaV₂(PO₄)₃, LiTiPO₅, LiVMoO₆, LiVWO₆, LiVP₂O₇ and LiFeAs₂O₇; and wherein the second crystalline substance includes one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆.
 6. The method of claim 1, wherein the second crystalline substance can be produced by heating the oxide compounds of A, M and N at molar ratio of a:b:c at about 400 to 1000° C. for about 8 to 15 hours.
 7. A method of providing a lithium secondary battery comprising: providing electrodes having wounded or stacked cathode, anode and divider film, the cathode comprising the cathode active material of claim 1; and providing a battery case and electrolyte, the electrodes and electrolyte adaptable to being sealed within the battery case.
 8. A method of preparing a cathode active material for lithium secondary batteries comprising: providing a first crystalline substance having a combination of lithium, iron and phosphorous sources, wherein the lithium, iron and phosphorous (Li:Fe:P) sources have molar ratios of about 0.95-1.1:1:0.95-1.1; providing a second crystalline substance having one or more members with the general formula A_(a)M_(b)N_(c)O_(d), wherein: A, M and N are different metals selected from groups IIA, IIIA, IVA, VA, IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table; 0≦a≦6, 0≦b≦6, 0<c≦6 and 0<d≦12; wherein a and b cannot both be 0 at the same time; and sintering the two crystalline substances at about 500 to 800° C. for about 5 to 32 hours to provide a mixed crystal, the mixed crystal having electrical conductivity of about 0.01 to 10 S/cm at about 25° C.
 9. The method of claim 8, wherein the phosphorous source and the second crystalline substance has molar ratios of about 1 to 0.01-0.05, taking only the phosphorous source in the material into consideration.
 10. The method of claim 8, wherein the second crystalline substance can be produced by heating the oxide compounds of A, M and N at molar ratio of a:b:c at about 400 to 1000° C. for about 8 to 15 hours.
 11. The method of claim 8, wherein the second crystalline substance includes one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆.
 12. The method of claim 8, wherein the lithium source includes one or more members selected from the group consisting of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate; the iron source includes one or more members selected from the group consisting of ferrous oxalate, ferrous carbonate, iron acetate, iron oxide, iron phosphate, iron pyrophosphate and iron nitrate; and the phosphate source includes one or more members selected from the group consisting of ammonium phosphate, ammonium dihydrogen phosphate, ammonium, iron phosphate, phosphoric acid and lithium dihydrogen phosphate.
 13. The method of claim 8, further comprising sintering a carbon additive with the two crystalline substances, the carbon additive capable of providing the mixed crystal with about 1-5% of carbon by weight.
 14. The method of claim 13, wherein the carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol.
 15. A method of providing a lithium secondary battery comprising: providing electrodes having wounded or stacked cathode, anode and divider film, the cathode comprising the cathode active material of claim 8; and providing a battery case and electrolyte, the electrodes and electrolyte adaptable to being sealed within the battery case.
 16. A method of preparing a cathode active material for lithium secondary batteries comprising: providing a first crystalline substance having one or more members selected from the group consisting of LiFePO₄, LiMnPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃, Li₂NaV₂(PO₄)₃, LiTiPO₅, LiVMoO₆, LiVWO₆, LiVP₂O₇ and LiFeAs₂O₇; providing a second crystalline substance having one or more members selected from the group consisting of Bi₄Ti₃O₁₂, CuNb₂O₆, MnTaO₄, FeWO₄, ZnZrNb₂O₈, NiNb₂O₆, NiZrNb₂O₈, FeTiNb₂O₈, MnTiNb₂O₈, MgSnNb₂O₈, ZnTa₂O₆, Cu_(0.85)Zn_(0.15)Nb₂O₆, YBa₃Ti₂O_(8.5), Zr_(0.75)Ti_(0.75)Sn_(0.5)O₄, HfTiO₄ and MgNb₂O₆; and sintering the two crystalline substances at about 500 to 800° C. for about 5 to 32 hours to provide a mixed crystal, the mixed crystal having electrical conductivity of about 0.01 to 10 S/cm at about 25° C.
 17. The method of claim 16, further comprising sintering a carbon additive with the two crystalline substances, the carbon additive capable of providing the mixed crystal with about 1-5% of carbon by weight.
 18. The method of claim 17, wherein the carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol.
 19. The method of claim 16, wherein the second crystalline substance can be produced at temperatures of about 400 to 1000° C. for about 8 to 15 hours.
 20. A method of providing a lithium secondary battery comprising: providing electrodes having wounded or stacked cathode, anode and divider film, the cathode comprising the cathode active material of claim 16; and providing a battery case and electrolyte, the electrodes and electrolyte adaptable to being sealed within the battery case. 